1. Technical Field
The invention relates to current sensing devices and more particularly to a leadless current sensor using Hall Effect detectors.
2. Description of the Prior Art and Problem
U.S. Pat. No. 4,887,027 to Strasser describes a leadless current sensing probe which, when placed near a current carrying conductor, generates signals from which the probe's angle of orientation and distance from the conductor can be determined. Strasser teaches a probe for direct current (D.C.) electrical systems constructed using two Hall Effect plates, one placed forward of the other in a probe. It is anticipated that in use the end of the probe where the forward Hall Effect plate is located is directed toward the current carrying conductor. The Hall Effect plates have a common center axis, with the rearward plate rotated on the axis relative to the forward plate. The distance between the plates is treated as a known.
Hall Effect devices measure magnetic flux density by exhibiting a voltage across the devices proportional to the local magnetic flux density or by exhibiting changes in resistance as a function of magnetic field flux density (magneto resistive devices). It is well known that a wire carrying a current has a magnetic field associated therewith. If the wire can be treated as being linear and infinite in length the relationship between the current carried by the wire and the magnetic field flux density B at a distance r to the wire is:B=μI/2πr  (Strasser equation (18))(1)where:    μ is the permeability of the medium surrounding the conductor, with the permeability for vacuum after being the default valve,    I is the current in the conductor, and    r is the minimum perpendicular distance to the wire.Strasser rearranges equation (1) to derive the relationship:I=B2πr/μ  (Strasser equation (32))(2)From this relationship Strasser concludes that once “r” or distance is determined “I” or current can be solved for using measured magnetic flux. Strasser is able to determine distance by comparison of two voltage measurements taken across each of the pair of Hall Effect plates, a total of four measurements.
Current sensors are essential components in contemporary control systems for motor vehicles. Such sensors are preferably inexpensive and should not require leads connected to the wire in which current is to be measured. The Strasser device and other Hall Effect based current measuring devices provide leadless current measurement. However, many of the devices, other than Strasser, have required careful control, and prior knowledge of, the distance separating the Hall Effect based probe and the conductor. The Strasser device, though tolerant of changes is spacing between the probe and wire, still treats determination of the distance between wire and probe as a necessary step before determining the conductor current.